Problem: $ { {2} \times \left[\begin{array}{rr}{2} & {1} \\ {1} & {-1}\end{array}\right] = {?} } $
Explanation: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{2}\times{2} & {2}\times{1} \\ {2}\times{1} & {2}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{4} & {2} \\ {2} & {-2}\end{array}\right]}$